What Should "Normal" compression be for a used engine?

[n0uy]

I got this 81 about a month ago. When I got it home the crankcase was full of gas. The previous owner had tried to get it running in preparation for selling it. I am concerned that in the process he may have scored the cylinders a bit. On the left side I get 78 psi dry. On the right I get about 85 dry. Put a shot of oil in each spark plug hole and cranked it over a couple times. Now both sides have about 110 psi which would indicate ring bypass. My question is what is normal compression on a used engine of this type?

Thanks for any replies. ray

 

[Wolfe_11B]

That's pretty low. What brand gauge, and how are you exactly performing the test?

 

[xschris]

Make sure to hold the throttle fully open when testing. A new motor should be around 150-160 psi. The minimum is around 115. My 79 that has 33k+ miles and still has 150psi on the original pistons rings and valves. At 25k I put a new set of gaskets in the top end and lapped the valves. The only reason I did it was because the head had a slight oil weep.

 

[n0uy]

This is a concern of mine that there could be some damage. I used a accurate compression gauge that screws in the plug hole and is sealed with an o-ring so it should be reliable. It has sat idle for at least 5 years but I really don't know much else about its history. The last itme it was licensed was 2010 but I think they had trouble getting it to run so it may actually have been much longer. When I finish the carb cleaning (again) I will warm it up and see how it tests at operating temp. If push comes to shove, where is a likely place to find decent parts as necessary? I am anixous to have it running to find out what other issues may be hiding. Obviously tires are required. These are weather cracked big time. esp in the rear.

BTW I did not open the throttle at all when performing the test so I probably choked the test a bit. I will do it again while the carbs are off that way no restrictions to the inlet. ray

 

[Wolfe_11B]

It makes a big difference with the throttle. Also when engines sit for a long time the rings have a possibility of sticking to the piston instead of the wall. Try letting some atf sit in each cylinder for a day, then ride it good for a bit and recheck. No sense in taking the thing apart if you don't have to

 

[n0uy]

OK, I did the compression test with the carbs removed today after getting home from work. They both averaged 125 psi so I feel better about that. I am waiting on carb gaskets for the bowl this time so it will be a few days before it goes back together. After it gets running smoothly I will sync the carbs, or at least check to make certain they are both equal. I have a couple of good gauges to do this. I found a back tire so that issue will be resolved soon also.

Was the mag aluminum wheel pretty common on the xs400s? It uses tubeless tires also both front and rear. Are the mag rims capable of using inner tubes if down the line I don't get tubeless tires?

 

[Wolfe_11B]

yep you can always run a tube in a tubeless rim

 

[n0uy]

OK, That's good to know. I wasn't sure if a rim made for tubeless tires was finished well enough on the inside so it wouldn't shred a tube when installed. I haven't removed the tire yet obviously.

 

[n0uy]

Just a quick update on my 81 xs400s. I got the rest of my carb parts awhile back and put in the new needle and seat along with bowl gaskets. New plug wires installed and the bike runs good. Made a couple trips to work with it (about 30 miles a day) and all is well.

Then I decided to go to the pump for gas fillup and put in the 91 octane option. In less than a mile the engine started running better than ever. It idled like it is supposed to, no real hesitation on throttle up from idle, and crusing down the street or highway has never been so smooth. This simple change has made me love this old classic even more.

Next job is to replace the rear brake shoes because when I changed the rear tire I discovered they need replacement. Already have the replacement parts so this weekend I will do that. Looking forward to a nice 4th of July holiday and plan to put on some miles. It is a bit chilly here in Northern MN at 5:30AM when I head to work so it will be a nice change to ride in the heat of the day for fun. Thanks for all the good advice guy and gals.

 

[Wolfe_11B]

With the low compression ratio on these engines, using 91 octane is a bit of a waste. Use the lowest you can find. Any difference you are seeing in using higher octane fuel is either all in your head or something else made it run better.

 

[xschris]

All the xs400's call for a 91 oct. min. 9.3:1 is not very low or super high. A lot of bikes run in the 8:1-5 range. The xs360 runs a 9.6:1 and calls for a 91 oct. Will a bike run on 86 oct? Sure, but I prefer 93

 

[bcware]

I only use premium. Not for increased power; I just feel like it is cleaner and the cost difference for such a small bike is negligible for me.

Have also never had any deposits or issues storing bikes with premium fuel in the tank over the winter and I do not use any additives. We have a long winter in NH too.

 

[Wolfe_11B]

Meh, 9.3 to 1 won't even be close to detonation on low octane pump gas.

 

[rescue50]

I don't know what it's like in the USA, but here in Canada the lower rated (87/89 octane) usually have up to 10 percent ethanol content. ive been told to avoid running ethanol in carbureted engines of any kind.

 

[Wolfe_11B]

Most places here its in all gas, regardless of octane. Exception being race, aviation, and marine fuel.

Also, all pump gas at gas stations is the same additive wise. Seems like some think that the higher grade is different fuel. It's not, it just has a higher octane rating. And all the grades in between the high and low at the pump are simply blended at the pump via valves. The station only gets 2 grades, unless you are getting specialized fuel.

 

[n0uy]

Wolfe 11B I assure you I am not imaging the improved running with the higher octane from my local gas station. Blended crap or not. It is possible that after sitting dormant for 5 plus years the engine has cleared itself somewhat after a 200 miles of running but it still ran a little crappy even after doing a lot of work to it. It seems unlikely to me that the improved running is just a coincidence to the gas change.

I didn't mean to start a debate about octane and I guess I will keep my comments to myself from now on. I am older than most of you guys I suspect and I am pretty content with keeping this bike mostly stock. There is a lot of traffic on this site about customizing and modifying the stock bike to something tricked out for various needs and wants. I guess I am not interested in that at all. This bike is pretty cool right out of the box as far as I am concerned.

 

[Kate on Wheels]

nOuy, I am new to the group. I have an '81 400 Special and I'm older too, and I just found this wonderful forum and I urge you not to keep your comments to yourself. I have really been enjoying reading about your work on your bike. I think these are some of the most beautiful motorcycles ever built, and I agree completely that they are absolutely cool right out of the box!! I couldn't have said it better. I see that your last post was about 3 years ago. I hope that you have not become discouraged. Have you been to a Yamaha dealer lately? My favorite dealer in Denver has a brand new 400, they are such great bikes they are making them again! Only they do not have starters. I'm old enough now I really like my starter! So, everyone, as to my reason for joining, my bike seems a little low on power. I will follow the advice in this thread and do a compression test on her, and let you all know.

 

[spectra]

nOuy, I am new to the group. I have an '81 400 Special and I'm older too, and I just found this wonderful forum and I urge you not to keep your comments to yourself. I have really been enjoying reading about your work on your bike. I think these are some of the most beautiful motorcycles ever built, and I agree completely that they are absolutely cool right out of the box!! I couldn't have said it better. I see that your last post was about 3 years ago. I hope that you have not become discouraged. Have you been to a Yamaha dealer lately? My favorite dealer in Denver has a brand new 400, they are such great bikes they are making them again! Only they do not have starters. I'm old enough now I really like my starter! So, everyone, as to my reason for joining, my bike seems a little low on power. I will follow the advice in this thread and do a compression test on her, and let you all know.

Why does it seem low on power? These are just a 400 and not super bikes.......

The new 400s are nice but a single cylinder thumper......kickstart isnt that bad really. I do actually use mine every so often.

Welcome to the forum !

 

[NewHavenMike]

Best thing you can do is a full tune up and see what happens. There are some very important steps to a tune up that often get overlooked and thats not counting non-engine related tasks.